
Originally Posted by
clinically.dead
John got shot in the shoulder and was invalided out. His injury was actually not life-threatening. He started missing all the action and the thrill of the hunt and thus developed psychosomatic limp and tremor. As Mycroft said " You're not haunted by the war, Dr Watson. You miss it." So once John got some excitement from the car chase all over London, the psychosomatic tendencies went away.
Basically, Sherlock was trying to prove that John had no physical problem with his leg, and the limp was caused by his mental state.